In the past, a known washing machine determines an amount of washings in a washing drum as a load at the beginning of a washing process, and controls the washing operation according to a result of the determination (e.g., patent literature 1). In the washing machine disclosed by the patent literature 1, when a start button is pressed, an impeller arranged inside a washing drum rotates for a specified time, a number of pulses is measured and is compared with reference data for load determination to determine if the load is extremely small, small, low, medium or high, such that the washing operation is controlled. Thus, for the washing machine of the patent literature 1, the larger the amount of the washings in the washing drum are, the more difficult the impeller is to rotate; and the load is determined according to the change of the number of pulses.